Connection count metric can be less than the real value

[YangKeao]

This issue has similar reason with #42761, because the conn.Close() will be called multiple times 🤦.

func (s *Server) onConn(conn *clientConn) {
...
	defer func() {
		terror.Log(conn.Close())
		logutil.Logger(ctx).Debug("connection closed")
	}()
...
}

And

func (cc *clientConn) Run(ctx context.Context) {
	defer func() {
		...
		if cc.getStatus() != connStatusShutdown {
			err := cc.Close()
			terror.Log(err)
		}

		close(cc.quit)
	}()

We have two ways to solve this problem:

1. Don't include any logic in conn.Close(). Only have logic in the sync.Once inside the closeConn.
2. Enhance the code, don't close the connection for multiple times! (it's more intuitive for the developers).

[YangKeao]

ref #32111

[YangKeao]

Also, #49424 didn't consider the situation that the resource group name may change 😠 .

[seiya-annie]

/found community